Trucks, boats, automobiles, and other vehicles are commonly equipped with various signal communication devices such as radios for receiving broadcast radio frequency (RF) signals, processing the RF signals, and broadcasting audio information to passengers. Satellite digital audio radio (SDAR) services have become increasingly popular, offering digital radio service covering large geographic areas, such as North America. These services receive uplinked programming which, in turn, is rebroadcast directly to digital radios that subscribe to the service. Each subscriber to the service generally possesses a digital radio having a receiver and one or more antennas for receiving the digital broadcast.
In satellite digital audio radio services systems, the radio receivers are generally programmed to receive and decode the digital data signals, which typically include many channels of digital audio. In addition to broadcasting the encoded digital quality audio signals, the satellite service may also transmit data that may be used for various other applications. The broadcast signals may include advertising, information about warranty issues, information about the broadcast audio programs, and news, sports, and entertainment programming. Thus, the digital broadcasts may be employed for any of a number of satellite audio radio, satellite television, satellite Internet, and various other consumer services.
In vehicles equipped for receiving satellite-based services, each vehicle generally includes one or more antennas for receiving the satellite digital broadcast. One example of an antenna arrangement includes one or more antennas mounted in the sideview mirror housing(s) of an automobile. Another antenna arrangement includes a thin phase network antenna having a plurality of antenna elements mounted on the roof of the automobile. The antennas(s) may be mounted at other locations, depending on factors such as vehicle type, size, and configuration.
As the antenna profiles for the satellite-based receiving systems become smaller, performance of the antenna may be reduced. To regain this lost performance, multiple small directional antennas may be used that compliment each other. This type of antenna system relies on switching to the best antenna source for the signal reception. Another option is to combine the antenna with beam steering electronics. For low cost applications, a switched diversity antenna may be employed. In doing so, the RF receiver typically controls which antenna to use by detecting the presence of a desired signal.
Systems employing more than one antenna generally switch to another antenna when the signal from the current antenna is lost, or when the system determines that another antenna has a stronger signal. In a moving vehicle with frequently changing antenna orientations, it is often desirable to switch frequently and quickly among the various system antennas. When the system switches from one antenna to another, the system must acquire the new signal and process it to extract the audio or other data that is being transmitted. However, switching randomly causes the digital demodulator to quickly detect a new signal with an unknown phase. While the phase detector circuitry of many digital receiver demodulators will track the phase to a given position, the resulting data orientation generally will be unknown. Because of the unknown data orientation, it is not possible to correctly interpret the transmitted data.
The unknown phase/orientation problem discussed above can be resolved by transmitting a known data sequence into the data stream at predetermined times. This data sequence can be referred to as a synchronization signal, a pre-amble, or frame synchronization pre-amble (FSP). By first decoding the synchronization or pre-amble bits sent as part of the transmitted signal, the receiver can accurately decode the audio or other data that has been transmitted, and can reproduce that data for the user. However, the decoding of the synchronization bits must occur quickly in order to avoid a delay in the decoding of the audio or other transmitted data. This is because a delay in the data decoding may result in a loss of data, which in turn can result in audio mute for radio applications. To avoid this condition, synchronization data generally needs to be transmitted and received/decoded soon after a switch has been made to a new antenna.
Although some current satellite transmission/reception schemes provide for periodic transmission of synchronization bits to allow a receiver to ultimately decode transmitted data, the frequency of transmission of these synchronization bits is often too slow to allow for use in fast diversity switching antenna systems where rapid switching among antennas is required in order for the system to be effective. It is therefore desirable to provide for a transmission and reception system that provides for enhanced transmission and reception of synchronization information.